RADIATION 213 of clear ice is cut into the form of a large burning-glass it can be employed to inflame tinder by concentrating the sun's rays, and the lens does the work nearly as rapidly as if it had been made of glass. It is certainly not what we ordinarily call " heat " which can be transmitted under conditions like these. Radiation is undoubtedly a trans- ference of energy, which was in the form commonly called heat in the radiating body, and becomes heat in a body which absorbs it ; but it is transformed as it leaves the first body, and retransformed when it is absorbed by the second. Until the comparatively recent full recognition of the con- servation and transformation of energy it was almost im- possible to form precise ideas on matters like this ; and, consequently, we find in the writings even of men like Prevost and Sir J. Leslie notions of the wildest character as to the mechanism of radiation. Leslie, strangely, re- garded it as a species of " pulsation " in the air, in some respects analogous to sound, and propagated with the same speed as sound. Prevost, on the other hand, says, " Le calorique est un fluide discret; chaque element de calo- rique suit constamment la meme ligne droite, tant qu'aucun obstacle ne 1'arrete. Dans un espace chaud, chaque point est traverse sans cesse en tout sens nar des filets de calorique." 4. The more intensely the cannon ball is heated the more luminous does it become, and also the more nearly white is the light which it gives out. So well is this known that in almost all forms of civilized speech there are terms corresponding to our "red-hot," "white-hot," &c. As another instance, suppose a powerful electric current is made to pass through a stout iron wire. The wire becomes gradually hotter, up to a certain point, at which the loss by radiation and convection just balances the gain of heat by electric resistance. And as it becomes hotter the amount of its radiation increases, till at a definite temperature it becomes just visible in the dark by red rays of low refrangibility. As it becomes still hotter the whole radiation increases ; the red rays formerly given off become more luminous, and are joined by others of higher refrangibility. This process goes on, the whole amount of radiation still increasing, each kind of visible light becoming more intense, and new rays of light of higher refrangibility coming in, until the whole becomes white, i.e., gives off all the more efficient kinds of visible light in much the same relative proportion as that in which they exist in sunlight. When the circuit is broken, exactly the same phenomena occur in the reverse order, the various kinds of light disappearing later as their refrangibility is less. But the radiation continues, grow- ing weaker every instant, even after the whole is dark. This simple observation irresistibly points to the con- clusion that the so-called " radiant heat " is precisely the same phenomenon as " light," only the invisible rays are still less refrangible than the lowest red, and that our sense of sight is confined to rays of a certain definite range of refrangibility, while the sense of touch comes in where sight fails us. Sir W. Herschel in 1798, by placing the bulb of a thermometer in the solar spectrum formed by a flint-glass prism, found that the highest temperature was in the dark region outside the lowest visible red, a result amply verified at the time by others, though warmly contested by Leslie. 5. This striking conclusion is not without close ana- logies in connexion with the other senses, especially that of hearing. Thus it has long been known that the " range of hearing" differs considerably in different individuals, some, for instance, being painfully affected by the chirp of a cricket, which is inaudible to others whose general hearing is quite as good. Extremely low notes, on the other hand, of whose existence we have ample dynamical evidence, are not heard by any one ; when perceived at all they are /eft. 6. We may now rapidly run over the principal facts characteristic of the behaviour of visible rays (see LIGHT), and point out how far each has been found to characterize that of so-called " radiant heat " under similar conditions. (a) Rectilinear propagation : an opaque screen which is placed so as to intercept the sun's light intercepts its heat also, whether it be close to the observer, at a few miles from him (as a cloud or a mountain), or 240,000 miles off (as the moon in a total eclipse). (6) Speed of propagation : this must be of the same order of magni- tude, at least, for both phenomena, i.e., 186,000 miles or so per second ; for the sun's heat ceases to be percep- tible the moment an eclipse becomes total, and is perceived again the instant the edge of the sun's disk is visible. (c) Reflexion : the law must be exactly the same, for the heat-producing rays from a star are concentrated by Lord Rosse's great reflector along with its light, (d) Refraction : when a lens is not achromatic its principal focus for red rays is farther off than that for blue rays ; that for dark heat is still farther off. Herschel's deter- mination of the warmest region of the spectrum ( 4 above) is another case in point, (e) Oblique radiation : an illumi- nated or a self-luminous surface appears equally bright however it is inclined to the line of sight. The radiation of heat from a hot blackened surface (through an aperture which it appears to fill) is sensibly the same however it be inclined (Leslie, Fourier, Mellorii). (/) Intensity : when there is no absorption by the way the intensity of the light received from a luminous point-source is inversely as the square of the distance. The same is true of dark heat. But this is not a new analogy ; it is a mere consequence of (a) rectilinear propagation, (g) Selective absorption : light which has been sifted by passing through one plate of blue glass passes in much greater percentage through a second plate of the same glass, and in still greater percentage through a third. The same is true of radiant heat, even when the experiment is made with uncoloured glass ; for clear glass absorbs certain colours of dark heat more than others (De Laroche, Melloni). (h) Interference bands, whether produced by two mirrors or by gratings, characterize dark heat as well as light ; only they indicate longer waves (Fizeau and Foucault). (i) Polarization and double refraction : with special apparatus, such as plates of mica split by heat into numerous parallel films, the polarization of dark heat is easily established. When two of these bundles are so placed as to intercept the heat, an unsplit film of mica interposed between them allows the heat to pass, or arrests it, as it is made to rotate in its own plane (Forbes), (j) By proper chemical adjustments photographs of a region of the solar spectrum beyond the visible red have been obtained (Abney). We might mention more, but those given above, when considered together, are conclusive. In fact (b) or (i) alone would almost settle the question. 7. But there is a superior as well as an inferior limit of visible rays. Light whose period of vibration is too small to produce any impression on the optic nerve can be degraded by fluorescence (see LIGHT) into visible rays, and can also be detected by its energetic action on various photographic chemicals. In fact photographic portraits can be taken in a room which appears absolutely dark to the keenest eyesight. By one or other of these processes the solar spectrum with its dark lines and the electric arc with its bright lines have been delineated to many times the length of their visible ranges. The electric arc especially gives (in either of these ways) a spectrum of extraordinary length ; for we can examine it, as we can not examine sunlight, before it has suffered any sensible absorption.
